1. Field of the Invention
The present invention relates generally to a hydraulic process and apparatus for recovering electrical energy to use in motor test stations and more particularly to such a process and apparatus utilizing an asynchronous electric motor and a pump.
2. Discussion of Prior Art
Thermal motors, particularly high power motors, require operational and/or endurance testing, both of which are generally performed after the motors leave the assembly line.
These tests, to be representative of the conditions of use, must be carried out at variable speeds and moments, the braking moment being generally obtained by means of a frictional brake or an electromagnetic brake.
The energy furnished by the motors thus tested corresponds to the efficiency of the motors and to the energy spent, and is, in general, removed by cold water circulation and thereby lost.
It is thus clear that a considerable amount of energy is wasted in this fashion, and there is therefore a need for a method and apparatus for recovering at least a portion of this energy.
However, it is particularly difficult, for obvious reasons, to re-use the recovered energy on location, e.g., in the form of heat, and, it is known that a particularly beneficial means for recovering this energy consists of transforming the energy into electrical energy in a manner so as to be able to reinject it directly into the electrical network. A suitable process and apparatus for carrying out the process to obtain the above result must satisfy the following criteria;
(a) simplicity of installation and maintenance;
(b) reliability;
(c) a wide range of operational speeds (in a ratio of about 10:1);
(d) include a couple or torque which is adjustable at will and independent of the speed;
(e) low cost;
(f) high efficiency; and
(g) a good quality of electric current generated, i.e., having a frequency sufficiently fixed and precise and free of harmonics so as to be able to be reinjected without inconvenience into the electrical network.
There presently exist a number of techniques whose aim is to transform the mechanical power of a motor having a variable speed into electric power of fixed frequency.
To obtain this result, one purely electrical process comprises a frequency converter associated with an alternator. This process is, however, unacceptable, because the electrical current thus generated includes substantial harmonics which disturb the network in an unacceptable fashion.
Another electrical process consists of utilizing an asynchronous generator with injection of alternating current having a variable frequency.
Other processes consist of transmitting the mechanical power of the motor shaft, which turns at a variable speed, to a receiver shaft, which turns at a fixed speed or at a quasi-fixed speed, and to then couple the receiver shaft to an alternator or to an asynchronous generator connected to the electric network. To obtain a fixed speed of the receiver shaft, the following techniques are known: varying the mechanical speed, Ward Leonard type systems, and hydraulic systems having hydrostatic transmissions.
However, none of the above known processes can simultaneously satisfy the various criteria stated above.